Attorneys general from four states including Massachusetts have sharply questioned assurances by the Nuclear Regulatory Commission that the storage of spent nuclear fuel in America is safe.

The question of how the U.S. should store its nuclear waste is looming larger now that Washington has abandoned plans to store the spent nuclear fuel at Yucca Mountain in Nevada.

“Massachusetts has a strong interest and concern regarding storage of spent nuclear fuel at the Pilgrim nuclear power plant in Plymouth,” state Attorney General Martha Coakley wrote in a 143-page document submitted to the NRC.

Coakley argued that when the federal agency first licensed the 685-megawatt nuclear plant for operation in 1972, spent nuclear fuel was only supposed to be stored there temporarily and then transported offsite.

“Four decades later, the spent fuel continues to accumulate onsite at Pilgrim, posing a risk of catastrophic fire and other adverse environmental impacts,” she wrote.

Coakley and attorneys general from Connecticut, New York and Vermont said the Nuclear Regulatory Commission has failed to address the environmental impact of nuclear-waste storage in the face of reactor shutdowns, natural disasters such as earthquakes and floods, and after owners close down power plants and leave waste to sit there indefinitely.

“This proposed rule again fails to address secure alternatives or solutions,” said Jillian Fennimore, a spokeswoman for Coakley.

The NRC estimates that more than 70,000 metric tons of spent nuclear fuel has accumulated in the U.S., with about three-fourths of it still stored and cooled in 40-foot-deep pools at nuclear power plants.

Pilgrim has more than 500 metric tons of spent fuel – or 3,222 fuel assemblies – in pools. The plant is licensed for only 3,859 fuel assemblies.

By the end of 2014, Entergy Corp., the Louisiana-based company that owns Pilgrim, will transfer some of the spent fuel from the wet pools into three dry casks made of concrete and steel and widely considered to be a safer method of storing nuclear waste.

Each 18-foot-tall cask weighs 360,000 pounds when loaded and can hold 68 fuel assemblies, Entergy officials said. If the casks are filled to capacity with spent fuel rods, then Pilgrim will have shifted about 6 percent of its total spent fuel into the dry casks next year.

Page 2 of 2 - Dave Lochbaum, a nuclear engineer and the director of nuclear safety at the Cambridge-based Union of Concerned Scientists, wrote in his blog on Christmas Eve that his biggest concern regarding spent-fuel storage is what happens in the decades after a plant is closed if radioactive water leaks from the pools.

“The NRC had concluded that any leaks would be readily detected before the water migrated offsite to cause harm,” Lochbaum wrote.

But he argued that “regulatory oversight shrinks dramatically after reactors permanently shut down,” and that the NRC has already allowed the owners of one closed-down nuclear plant in Florida to sidestep requirements for monitoring spent-fuel pool levels.

Christopher Burrell may be reached at cburrell@ledger.com. Follow him on Twitter @Burrell_Ledger.